Achievements

How are the numbers of players estimated?Percent of players with achievement = (Number of players with achievement) / (All Players)
The estimation process deduces the number of All Players based upon the percentage of players that have each achievement and the constraint that the numbers of players should be integers.

Percent of players with achievement (e.g. 1.80020272731781006) is provided by Steam through a public API, but both Number of players with achievement and All Players are unknown. However, All Players values should be intergers (1.5 players wouldn't make sense), and should be within a range established by other factors such as the number of player reviews.

Why are there fractional numbers of players?
With a strict criteria of all integer player counts, some games produce no suitable results. Therefore, the process allows for a certain amount of error and thus fractional players.